1. Field of the Invention
The present invention relates to a thin-shaped hydrodynamic bearing for supporting a shaft by a dynamic pressure generated in lubricating fluid, and to a spindle motor having such a hydrodynamic bearing for use in such as a recording and/or reproducing apparatus.
2. Description of the Related Art
A hydrodynamic bearing (or referred to as “fluid dynamic bearing” hereinafter) is conventionally widely used as a bearing for a spindle motor for driving a recording disk in a small-size information recording apparatus such as a hard disk drive. In recent years, there is a growing demand for reducing a hard disk drive in size and thickness. In order to achieve this demand, a spindle motor for driving a disk is also requested to be downsized and thinned. Therefore, it is necessary to reduce an axial dimension of a hydrodynamic bearing used in a spindle motor. Conventionally, a hydrodynamic bearing has generally two sets of radial dynamic pressure generating grooves formed therein. For improvement thereof, in order to obtain a hydrodynamic bearing having a smaller axial dimension, only one set of such radial dynamic pressure generating grooves are formed in a radial bearing for radially supporting a shaft, so that an axial length of the radial bearing is reduced. The Japanese Patent Publication No. 3424739 (Document D1) discloses a conventional motor which includes a hydrodynamic bearing of the foregoing type having only one set of radial dynamic pressure generating grooves. Meanwhile, the Japanese Patent Unexamined Laid-open Publication No. 2003-92867 (Document D2) discloses another conventional hydrodynamic bearing which has two sets of short radial dynamic pressure generating grooves formed therein. In this Document D2, the two sets of short radial dynamic pressure generating grooves are interspaced from each other between a sleeve and a shaft.
As a thickness of a motor is reduced, a rotor is generally reduced in weight, and a magnitude of disturbance is accordingly reduced. Therefore, a radial bearing stiffness demanded by a radial loading capacity is reduced as the motor is thinner. As far as the viewpoint mentioned above is concerned, it is easy to reduce the axial length of the radial bearing. However, the reduction of the bearing in length causes the following problems.
A radius of a recording disk mounted on a spindle motor is significantly larger than a radius of a bearing of the spindle motor in general. Further, the radius of the disk is significantly larger than an axial length (thickness) of the bearing. Therefore, when a recording head tracks an outer peripheral of the disk, a force acting to make the shaft tilted is applied to the shaft. This force is referred to as “moment load”. When the applied moment load exceeds a level which the hydrodynamic bearing can withstand, the shaft and the sleeve are in contact with each other to cause a vibration and vary a rotational speed of the motor, and further the shaft and the sleeve are worn away. It is noted here that a property of the hydrodynamic bearing which can withstand the moment load is referred to as “bearing angle stiffness”. The hydrodynamic bearing having a high bearing angle stiffness can withstand a large moment load.